Question about Puzzle Mat - 96839

There are 6 bottom pieces with 2 edge pieces each. So 12 pieces. When assembling with 2x3 floor parts, 10 edge pieces are needed. 2 remain.

But: the parts differ and cannot be used everywhere (different parts are needed at the "corners" than at the sides).

It has a border piece for all sides. That would have to be 10.